the number of spots on your tire that will be in contact with the ground when your feet are in skidding position. since the gear ratios mean that the wheel will turn more times than the cranks. let's say, 48x16=3:1, 3 turns of the wheel for every turn of the cranks, lining up one spot on the tire every time you skid, wearing that one spot down. 49x16 is a 3.1:1 gear ratio, lining up more spots around the tire every time you skid since the wheel is not turning evenly with the cranks. not gonna go into the math since i suck at it myself, but that ratio gives you 16 skid patches. the tire contacts the ground in 16 places when the cranks are in skidding position, but another ratio will affect the crank to tire relationship in a way that creates its own number of skid patches.

more skid patches=longer tire life, as wear is spread more evenly.